The city’s $2 million sidewalk upgrade has reached a snag after excavation for a signal-light support in front of 143 Main St. unearthed a corner of the basement of the building. The basement extends 12 feet from the building, something the work crew did not expect. The city will now hire an engineering company to locate underground basements and other impediments under the 10 blocks of sidewalk scheduled to be upgraded during the next few years. The survey is expected to cost $45,000.